1. Field of the invention
The present invention relates to an automatic white balance device used in such products as a video camera.
2. Description of the Prior Art
The mainstream in automatic white balance devices used in conventional video cameras is shifting towards internal measurement method devices that do not require an external sensor. The operation of the internal measurement device is based on the assumption that averaging all subject colors will result in an achromatic signal. More specifically, this means the average values of the red, blue, and green signals on the image plane will be equal. Of course, when any signal component of the total image is a single solid color occupying a large part of the screen area, this principle will not result in correct white balance adjustment. It is therefore necessary in these cases to substitute signal for the chromatic signal to obtain the correct white balance.
A conventional automatic white balance device comprises a difference circuit, comparator, gate circuit, chrominance signal integrator, microcomputer, and chrominance signal amplifier. The R and B chrominance signals are input to the difference circuit, the blue chrominance signal level is subtracted from the red chrominance signal level, and the (R-B) difference signal is output. This difference circuit output is input to the comparator, which compares the input with a predetermined threshold level to obtain and output a gate signal to the gate circuit. The threshold level is set so that the gate signal is on when the difference between the red and blue signals is not too great, i.e., when the video signal is determined to not be biased towards red or blue. The gate circuit relays the red and blue signals to the chrominance signal integrator based on the gate signal input from the comparator. The chrominance signals input to the color signal integrator are accumulated for one field in one screen, and the integral is then output. The microcomputer then computes the gain of each chrominance signal required for the ratio between the R and the B signal integrals output from the chrominance signal integrator to be 1:1, adjusts the gain of the chrominance signal multiplier, and thus outputs the white balanced signal.
With this configuration, however, the value of the (R-B) color difference signal will be low under low luminance conditions even with a chromatic image because the signal quantity itself is low. As a result, the conventional automatic white balance device will determine the image to be achromatic, use this data for the white balance adjustment, and will not, therefore, result in an accurate white balance.